Hardened inorganic refractory fibrous compositions

ABSTRACT

Inorganic fibers such as mineral wool are bonded in a coherent mass with a binder such as colloidal positively charged particles having a silica core and a coating of a polyvalent metal-oxygen compound. The binder is flocculated from solution onto the fibers by agents such as attapulgite or hectorite. A flocculated and bonded wet compact of this material is impregnated with negatively charged colloidal silica to produce a hard body.

Unitedv States Patent 1191 Weidman I Nov. 27, 1973 [75] Inventor: VerneWesley Weidman,

Wilmington, Del.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: May 3, 1972 [21] Appl. No.: 249,969

[52] US. Cl 106/65, 106/67, 106/68, 106/69 [51] Int. Cl C04b 35/10 [58]Field of Search 106/55, 65, 67, 68, 106/69 [56] References Cited UNITEDSTATES PATENTS 2,808,338 1011957 Bruno et a1. 106/69 3,077,413 2/1963Campbell 106/69 3,231,401

1/1966 Price et a1 106/65 3,253,936 5/1966 Weindel 106/65 3,296,060l/1967 Seitzinger 106/69 3,321,171 5/1967 Gorka et a1. 106/69 3,445,2505/1969 Preece 106/65 3,507,944 4/1970 Moore 106/65 3,629,116 12/1971Gartner et a1. 106/69 Primary Examiner-James E. Poer Att0rney-Richard H.Burgess [5 7] ABSTRACT Inorganic fibers such as mineral wool are bondedin a coherent mass with a binder such as colloidal positively chargedparticles having a silica core and a coating of a polyvalentmetal-oxygen compound. The binder is flocculated from solution onto thefibers by agents such as attapulgite or hectorite. A flocculated andbonded wet compact of this material is impregnated with negativelycharged colloidal silica to produce a hard body.

13 Claims, No Drawings HARDENED INORGANIC REFRACTORY FIBROUSCOMPOSITIONS BACKGROUND OF THE INVENTION This invention relates tocompositions primarily of inorganic fibers held in a coherent mass by abinder. These compositions are useful as insulation panels, molds andother structural elements which 'must be ca pable of withstanding hightemperatures. U.S. Pat. No. 3,100,734 teaches the use of colloidalsilica as a binder for refractory fiber structures. While the patentalleges the structures so made are suitable for temperatures as high as-2,300F several problems arise. It is difficult to deposit all of thecolloidal silica from solution onto the fibers. Thus some silica remainsin solution and is lost. Further, to impart additional strength to thearticles the patent teaches addition of an organic binder in addition tothe silica. This binder is burned off in use and the strength it impartsis lost.

U.S. Pat. No. 3,224,927 suggests the use of starch to cause silicabinders to precipitate ontothe refractory fibers. While this cutsdown onsilica use and provides adequate strength it also decreases the maximumtemperature at which the structure can be used to, about 1,000F. Animprovement in working temperature from 1,000 to 1700F was achieved bysubstitution of inorganic fibrous potassium titanate for. the starch asa flocculent. This flocculent was compatible with binders such aspositively charged colloids'having a dense silica core coated withapolyvalent metal-oxygen compound. The use of potassium titanate as aflocculant is taught in Defensive Publication 724,222. However, eventhis improved systemcould'not completely exhaust all of the binder ontothe fibers andthe resulting structure could not be used- 'at gas flametemperatures, 2,000.2,300F.

This invention represents an improvement over applicants copendingapplication Ser. No. 195,864 filed Dec. )1, 1971 which is concerned withsimilar but softer materials not impregnated with negatively chargedcolloidal silica.

SUMMARY OF THE, INVENTION Flocculated, totally inorganic fibrousrefractory compositions consisting essentially of 50 to 98 parts byweight fibers and 2 to 50 parts by weight binder plus flocculent wherethe weight ratio of binder to flocculent is from 3:1 to 1:5 and to 200%by weight negatively charged colloidal silica impregnant based on theweight of the fibers are useful for high temperature applications.Preferably, 30% to 150% impregnant is used to minimize migration of thecolloids. Binders for these structures are positively charged colloidalparticles that can be silica coated with a polyvalent metal-oxygencompound, boehmite alumina, amorphous fumed alumina or basic aluminumchloride. Suitable flocculents are negativelycharged clay-minerals suchas montmorillonite, saponite, hectorite and attapulgite. The termconsisting essentially of is intended to indicate the essentialcomponents of the compositions of this invention but is not intended toexclude other components whichcan be-added without detracting from theuse of the compositions of this invention as refractory articles.

These structures can be made by first forming a dilute'(l-5% solids)aqueous slurry of inorganic fibers,

and adding the positively charged binder. Usually the binder is added asan aqueous suspension. After the binder is mixed with the fiber slurrythe flocculent is added. The slurry with all ingredients added is mixedfor an additional 5 to minutes and the refractory objects are formedsuch as by vacuum on a screen. The wet-formed objects are thenimpregnated with negatively charged colloidal silica. The moist cakeformed can be dried as is or it can be shaped further by molding onforms, wrapping on mandrels or the like, and then dried. The negativelycharged impregnant greatly increases the strength and hardness of theobjects and prevents or minimizes migration of thecolloidal materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The refractory compositions ofthis invention have as their major constituent inorganic fibers. Theparticular inorganic fiber selected is not critical so long as it iscapable of withstanding temperatures at which the finished compositionis to be used. Fibers such as alumino silicates, mineral wool, fiberglass, asbestos and quartz can be used. In the process for making thecompositions of this invention the refractory fibers are charged asdilute aqueous slurries. To assure uniformity in the compositions thefibers should be as completely disaggregated as possible. Thus vigorousagitation of the fiber slurry is desirable.

The binders useful in this invention are all positively chargedinorganic colloids. Colloidal silica coated with a polyvalentmetal-oxygen compound, amorphous fumed alumina, boehmite alumina andbasic aluminum chloride are useful in this invention. The coatedcolloidal silica is available in the form of an aquasol of pH 2.5 to 7.Outside of this pH range the sols tend to be unstable. Negativelycharged colloidal silica is coated with a sufficient amount of apolyvalent metal-oxygen compound to impart a positive charge to thecoated parti- V cle. The-metals are present as oxides, hydroxides orhydrated oxides. The metals useful for preparing these sols have avalence of 3 or 4. Examples are aluminum which is the preferred metal,chromium, titanium and zirconium. These sols are described in Alexanderand Bolt, U.S. Pat. 3,007,878. The particle size is 2 to millimicronsand the sols are available in 5 to 50% solids concentrations. These solsmay be added to the slurry of refractory fibers either in theirconcentrated commercial form or diluted. This is the preferred binder.

Positive Sol 232 and Positive Sol M, both prodnets of E. I. du Pont deNemours and Company can be used as the binder. Positive Sol 232 is asilica sol stabilized by reacting with boric acid stabilized aluminumacetate as described in U.S. Pat. No. 3,620,978 Moore. Positive Sol 130M is an acidic aqueous disper sion of positively charged colloidalparticles consisting of a dense silica core coated with positivelycharged polymeric Al-(-Al) species, with a mole ratio of aluminum tosilica on the surface of about 1:1.

Alumina in several forms is a suitable binder. One is boehmite aluminamonohydrate. This product and its preparation are described in U.S. Pat.No. 2,915,475. Boehmite alumina is also a commercially available producthaving the name DISPAL. The alumina fibers described in the patent areon the order of from 100 to 1,000 millimicrons in length and on theorder of 10 millimicrons in diameter. Another useful form of alumina iscolloidal fumed alumina. These materials when used as binders may becharged to the fiber slurry as the commercial solid particulate matteror as an aqueous dispersion.

Basic aluminum chloride, Al OH Cl, is also a useful binder in thisinvention. While it would seem that this is not a positively chargedcolloid, when the described compound is dispersed in water it is presentas colloidal polymeric cations with the chloride ions in solution. Thismaterial is commercially available either as a solid or liquid.

Because of the efficiency of the flocculating agents of this invention,90 to 95 percent of the suspended binder is exhausted onto therefractory fibers. Thus the amount of binder charged to the slurryshould be 5 to percent greater than the amount desired in the finalcomposition. Flocculents of the prior art are unable to achieve thishigh rate of binder exhaustion, and thus their use requires that greateramounts of binder be used. This often causes increased viscosity anddifficult filtration of the slurry. For example, fibrous potassiumtitanate as a flocculent only exhausts about 85% of the binder in theslurry.

The flocculents useful in this invention are specific clay-type mineralswhich are negatively charged colloids. The particular materials whichare effective are montmorillonites, saponites, attapulgites andhectorites. By montmorillonite is meant a mineral of the approximateformula (Al,Fe -,Mg )Si,,O, ,(OH) Na,Ca More particularly, this mineralis a bentonite clay having a high degree of exchangeable sodiumassociated with it. Typical of this material is the type found in theBlack Hills area of the United States. The general formula shown aboveapproximately describes the Black Hills montmorillonite. Montmorillonitefound elsewhere may vary somewhat from the formula shown, and the termas used herein is not limited to minerals of the specific formula shownor to minerals from a specific geographical area. However, the generalformula and geographic source provide a convenient method of designatingthe type of mineral encompassed within the term montmorillonite.Montmorillonite is a preferred flocculent because it is inexpensive.-

Another useful flocculent is attapulgite, the principal mineral inattapulgus clay. Attapulgus clay is found in Georgia and Florida and inthe Ural mountains in the U.S.S.R. This mineral, like montmorillonite,is primarily a hydrated magnesium aluminum silicate. However, thecrystalline structure differs and, unlike the montmorillonite,attapulgite has a low level of associated exchangeable metal ions anddoes not swell in water. The formula for the ideal attapulgite is Ofcourse individual samples of attapulgite may vary from this generalformula depending on their source and treatment. Attapulgite isdescribed in detail in lndustrial and Engineering Chemistry 59 pp 59-69,Sept., 1967.

Another mineral useful as a flocculent in this invention is hectorite.This mineral is sodium magnesium fluorolithosilicate. The latticestructure is of the montmorillonite type but hectorite differs from thecommon montmorillonites in that it contains almost no aluminum. It issubstituted with lithium and has fluorine substituted for some hydroxylgroups in the lattice. This material has a high level of exchangeablesodium. It is found in the Mojave desert of California.

A fourth mineral useful as a flocculent in this invention is saponite.This mineral, like hectorite, has a montmorillonite type of lattice. insaponite magnesium is substituted for aluminum causing distortion of thelattice. Saponites from various locations and differing somewhat incomposition are described in Silicate Science, Vol. I, W. Eitel,Academic Press, 1964, pp 234ff. Saponite is the most effectiveflocculent.

Suitable negatively charged colloidal silica sols generally haveparticle sizes in the range of 4 to millimicrons and are oftenstabilized with lithium, sodium or potassium. Materials commerciallyavailable from Du Pont include Ludox HS4O having 40% solids by weightand Ludox TM. These products are of the types described in U.S. Pat. No.2,574,902. Lithium stabilized Polysilicate 85, with a mole ratio of SiO:Li O of 8.5 is of the type described in US. Pat. No. 2,668,149.

In addition to the basic ingredients of the composition of thisinvention, inorganic fillers may be included. Materials such as zircon,alumina bubbles, zirconia, mullite, vermiculite and perlite may beincluded by adding them to the dilute fiber slurry. Further, other typesof refractory materials such as silica may be coated onto the surface ofthe refractory structures of this invention. Care should be exercised sothat fillers which would detrimentally interact with the binder in thefiber slurry are not used. For example, a negatively charged silica solwould cause gelation of the positively charged binders of this inventionbefore the binder could be exhausted onto the fibers by the flocculent.

The totally inorganic structures of the compositions of this inventionmay be used at temperatures as high as 2,300F without smoking orsignificant strength loss or shrinkage. The compositions of the priorart using organic binders or flocculents could not be used at theseelevated temperatures.

The following examples further illustrate this invention. All parts andpercentages are expressed on a weight basis unless otherwise noted. Themodulus of rupture data was obtained by breaking test bars (dimensions 1inch x 1 inch x 6 inches as cut from the vacuum formed pads) on anlnstron Machine. Using a 4 inch span (distance between the two supportsunder the test bar) and a crosshead speed of 0.1 inch/min, the test barwas center loaded to failure in flexure. Modulus of rupture values werecalculated using the following formula:

R 3 Wl/2 bd where:

R modulus of rupture in lbs/inch W load in pounds at which the specimenfailed I distance (span) in inches between the centerlines of the lowerbearing edges b width of specimen in inches d depth of specimen ininches Example 1 details the process and the test results. Three othercompositions of the invention were prepared and tested as in Example 1,with the differences set forth in each example.

EXAMPLE 1 A dilute slurry is prepared containing 2-lbs. of CarborundumsFiberfrax aluminosilicate refractory bulk fiber in l3-gallons l08-lbs)water. To this slurry, 0.53- lb of Du Pont Positive Sol M (30% solids)an acidic aqueous dispersion of positively charged colloidal particlesconsisting of a dense silica core coated with positively chargedpolymeric alumina, is added with good stirring, which is continued forS-minutes. Next, 0.16-lb of mineral flocculent, which is a sodiumexchangeable bentonitic clay (Accoflod 350 from American ColloidCompany) is added to the slurry with good stirring in the form of a 6%solids, negatively charged colloidal suspension in water. After 15additional minutes of stirring, a test pad, with dimensions of 9 A inchdiameter X approximately 1 inch thick, is prepared in the slurry byvacuum-forming techniques. The mold containing the wet pad is thenwithdrawn from the slurry with vacuum still applied. After inverting themold as sembly to a rightside-up position, excess water is removed fromthe pad by the vacuum in about l-minute.

The damp pad, while still retained in the mold, is then saturated with a50/50 mixture of Du Pont Ludox HS 40 and Ludox TM by pouring 1 liter ofthis collodial sil-.

ica blend over the entire exposed surface of the pad. Excess Ludox isremoved from the pad by the suction, which is still maintained on themold. The silicaimpregnated pad is then removed from the mold andtransferred to a perforated stainless steel plate support. After the padis oven dried overnight in an air atmosphere at a temperature of about400F. test bars are cut from the pad and weighed. Strength, density, andshrinkage properties. of this composite product are then determined,with the results as given below:

Density 29-lbs/ft Fired (linear) shrinkage 2-hrs at 2lO0F 3% Modulus ofrupture dry 190 psi Fired 4-hrs at 1830F l37 psi Fired 2-hrs at 2lO0F 80psi EXAMPLE 2 A dilute slurry is prepared containing 2-lbs of Babcock &Wilcoxs Kaowool" aluminosilicate refractory bulk fiber in l3-gallonsl08-lbs) water. Into this slurry is stirred 0.26- lb of Du Pont PositiveSol 130 M. Next, 0.12-lb of Accofloc 350 mineral flocculent is stirredinto the slurry. After 15 additional minutes of stirring, a test pad isprepared.

The damp pad is then impregnated with 1 liter of a 50/50 mixture of DuPont Ludox H s-40 and Ludox TM, the excess Ludox is removed from the padby the suction, and the pad is oven dried. Strength, density, andshrinkage properties of test bars of this composite product are thendetermined with the results as given below:

Density 29-lbs/ft Fired (linear) shrinkage 2 hrs at 2lO0F 3% Modulus ofrupture dry 257 psi Fired 4-hrs at 1830F 168 psi Fired 2-hrs at 2lO0F l10 psi EXAMPLE 3 A dilute slurry is prepared containing 2-lbs ofFiberfrax fiber in l3-gallons (l08-lbs) water. Into this slurry isstirred 0.5 3-lb of Du Pont Positive Sol 232 (30% sol ids). Next,0.16-lb of Accofloc 350 mineral flocculent is stirred into the slurry.After additional minutes of stirring, a test pad is prepared.

The damp pad is then impregnated with 1 liter of Du Pont HS-40, theexcess Ludox is removed from the pad by the suction, and the pad is ovendried. Stength, density, and shrinkage properties of test bars of thiscomposite product are then determined with the results as given below:

Density 24-lbs/ft Fired (linear) shrinkage 2-hrs at 2lO0F 3.8% Modulusof Rupture dry 136 psi Fired 4-hrs at 1830F psi Fired 2-hrs at 2lO0F 83psi EXAMPLE 4 A dilute slurry is prepared containing 2-lbs of Fiberfraxfiber in l3-gallons (l08-lbs) water. Into this slurry is stirred O.26-lbof Du Pont Positive Sol M. Next, 0.12-lb of Accofloc 350 mineralflocculent is stirred into the slurry. After 15 additional minutes ofstirring, a test pad is prepared.

The damp pad is then impregnated with 1 liter of Du Pont Polysilicate85. Excess polysilicate is removed from the pad by the suction, and thepad is oven dried. Strength, density, and shrinkage properties of testbars of this composite product are then determined with the results. asgiven below:

Density 22-lbs/ft Fired (linear) shrinkage 2-hrs at 2lO0F 1.7%

Modulus of rupture dry 69 psi Fired 4-hrs at 1830F 93 psi Fired 2-hrs at2lO0F 123 psi What is claimed is:

l. A flocculated inorganic fibrous refractory composition consistingessentially of about 50 to 98 parts by weight inorganic refractoryfibers and from about 2 to 50 parts by weight positively charged binderplus negatively charged flocculent, the weight ratio of binder toflocculent being from 3:1 to 1:5, and about 10% to 200% by weightnegatively charged colloidal silica impregnant based on the weight ofthe fibers, said binder being selected from the group consisting ofpositively charged colloidal particles having a dense silica core and acoating of polyvalent metal-oxygen compound, colloidal amorphous fumedalumina, colloidal boehmite alumina and basic aluminum chloride, saidflocculent being selected from the group consisting of mont-,morillonite, saponite, hectorite and attapulgite.

. 2. The composition of claim 1 wherein the binder is a so] of particleshaving a silica core coated with a polyvalent metal-oxygen compound. I

3. The composition of claim 2 wherein the ratio of binder to flocculentis from 2:3 to 4:3.

4. The composition of claim 2 having from 75 to 90 parts by weightinorganic refractory fibers and 10 to 25 parts by weight binder plusflocculent.

5. The composition of claim 4 wherein the flocculent is montmorillonite.

6. The composition of claim 1 wherein from 30% to by weight impregnantis used based on the weight of the fibers.

7. The composition of claim 2 wherein the flocculent is a mixture ofmontmorillonite and saponite.

8. A formed object of the composition of claim 1.

9. In a process for binding inorganic refractory fibers with aflocculated binder wherein said binder is selected from the groupconsisting of positively charged colloidal particles having a silicacore coated with a polyvalent metal-oxygen compound, boehmite alumina,fumed alumina and basic aluminum chloride and the flocculent is selectedfrom the group consisting of montmorillonite, attapulgite, saponite andhectorite,

ried together and then filtered, the improvement comprising impregnatingthe wet filter cake with a negatively charged colloidal silicaimpregnant.

10. A process comprising the following steps:

a. preparing a slurry of inorganic refractory fibers, a

positively charged colloidal binder, and a negatively chargedflocculent,

b. filtering said slurry to form a wet compact on a filter,

0. impregnating said wet compact with a negatively charged colloidalimpregnant, and

d. drying the impregnated compact.

11. The process of claim 10 wherein the binder is selected from thegroup consisting of positively charged colloidal particles having adense silica core and a coating of polyvalent metal-oxygen compound,colloidal amorphous fumed alumina, colloidal boehmite alumina and basicaluminum chloride, the flocculent is selected from the group consistingof montmorillonite, saponite, hectorite and attapulgite, and theimpregnant is colloidal silica.

12. The process of claim 10 wherein the proportion of materials used are50 to 98 parts by weight fiber and 2 to 50 parts by weight binder plusflocculent wherein the weight ratio of binder to flocculent is from 3:1to 1:5, and wherein the impregnant is added in an amount of from 10% to200% by weight of the fibers.

13. The process of claim 12 in which from 30% to by weight of impregnantis used based on the weight of the fibers.

2. The composition of claim 1 wherein the binder is a sol of particleshaving a silica core coated with a polyvalent metal-oxygen compound. 3.The composition of claim 2 wherein the ratio of binder to flocculent isfrom 2:3 to 4:3.
 4. The composition of claim 2 having from 75 to 90parts by weight inorganic refractory fibers and 10 to 25 parts by weightbinder plus flocculent.
 5. The composition of claim 4 wherein theflocculent is montmorillonite.
 6. The composition of claim 1 whereinfrom 30% to 150% by weight impregnant is used based on the weight of thefibers.
 7. The composition of claim 2 wherein the flocculent is amixture of montmorillonite and saponite.
 8. A formed object of thecomposition of claim
 1. 9. In a process for binding inorganic refractoryfibers with a flocculated binder wherein said binder is selected fromthe group consisting of positively charged colloidal particles having asilica core coated with a polyvalent metal-oxygen compound, boehmitealumina, fumed alumina and basic aluminum chloride and the flocculent isselected from the group consisting of montmorillonite, attapulgite,saponite and hectorite, and wherein the fibers, flocculent and bindersare slurried together and then filtered, the improvement comprisingimpregnating the wet filter cake with a negatively charged colloidalsilica impregnant.
 10. A process comprising the following steps: a.preparing a slurry of inorganic refractory fibers, a positively chargedcolloidal binder, and a negatively charged flocculent, b. filtering saidslurry to form a wet compact on a filter, c. impregnating said wetcompact with a negatively charged colloidal impregnant, and d. dryingthe impregnated compact.
 11. The process of claim 10 wherein the binderis selected from the group consisting of positively charged colloidalparticles having a dense silica core and a coating of polyvalentmetal-oxygen compound, colloidal amorphous fumed alumina, colloidalboehmite alumina and basic aluminum chloride, the flocculent is selectedfrom the group consisting of montmorillonite, saponite, hectorite andattapulgite, and the impregnant is colloidal silica.
 12. The process ofclaim 10 wherein the proportion of materials used are 50 to 98 parts byweight fiber and 2 to 50 parts by weight binder plus flocculent whereinthe weight ratio of binder to flocculent is from 3:1 to 1:5, and whereinthe impregnant is added in an amount of from 10% to 200% by weight ofthe fibers.
 13. The process of claim 12 in which from 30% to 150% byweight of impregnant is used based on the weight of the fibers.